The purpose of this study is to determine the maximum tolerated dose and to gather toxicity information of R115777 when administered twice a day for 21 days in children who are refractory to standard therapy. another goal of this study is to determine the pharmacokinetics of R115777. Patients will take R115777 by mouth q12h for 21 days. There will be a 7 day rest period afterwards. The first three patients will be entered at the starting dose of 150 mg/m2 q12h. In the absence of a dose limiting toxicity (DLT), patients will be entered in cohorts of 3 at subseuqent dose levels until a DLT occurs. In the event a DLT occurs, additional patients will be treated at a lower dose and the maximum tolerated dose will be defined. Pharmacokinetic patients or their legal guardian must give informed consent prior to starting treatment.